xctool should use the action-specific build configurations from the xcscheme.

https://github.com/facebook/xctool/commit/9340e899a4f2d0c173358a46096c96c19cd0ca70 tells the whole story.

I'm trying to slowly remove all the reasons we call `-showBuildSettings` when _xctool_ starts.  Picking which configuration to use was one of those reasons.

This makes xctool behave just like Xcode in choosing which build configuration to use for each build action.  e.g. the Test action in Xcode would use the Debug configuration by default; and the Archive action in Xcode would use the Release configuration by default.